SPECIFIC GRA\^ITY AND DISPLACEMENT OF SOME SALINE SOLUTIONS. 203 



It is obvious that the method is applicable only to salts which have a mother- 

 liquor, such as KCl ; EbBr ; CaClaeHaO ; BaCl22H2U ; it is inapplicable to salts such as 

 CaClz ; BaClg ; and the like, which have no legitimate mother-liquor. 



§ 121. It is an essential condition of success that the work be carried on in a room, 

 for the time being, especially devoted to the purpose, and occupied by one investigator. 

 He must have in it everything that he requires, including his balance. The window of 

 the room must face the north, and the precautions generally to be observed are similar 

 to those prescribed by Bunsen for the practice of his original gasometric method. 



The salts used in this research were jhe chlorides, bromides, and iodides of 

 potassium, rubidium, and caesium. The rubidium and caesium preparations were from 

 the works of Schuchardt in Goerlitz, and on examination proved to be of the highest 

 degree of purity. The potassium salts were also unexceptionable as regards quality, 

 and were supplied by Merck. All of these salts dissolve easily, and most of them 

 abundantly, in water. They also crystallise with great readiness. 



§ 122. The first operation is to prepare a hot solution of the salt such that, after stand- 

 ing over nighb, or for such length of time as may be deemed sufficient, it shall furnish 

 about 60 c.c. of mother-liquor and about 1 5 c.c. of crystals. In the case of the potassium 

 salts there was no difficulty, as their solubility at all temperatures is well known. The 

 solubility of the rubidium and caesium salts had to be determined, at least approxi- 

 mately, in each case, in order to economise the costly material. The following simple 

 method furnished the required information easily and expeditiously. A suitable vessel, 

 beaker or flask, is weighed empty, and then with 25 grams of distilled water, of the 

 temperature of the air. The salt is then gradually added and the mixture stirred with 

 the thermometer. In the case of every one of these salts the temperature falls rapidly 

 while dissolving, and by as much as from 15° to 20°. The salt is added as rapidly 

 as it is taken up by the water. When the fall of temperature slackens, a minimum 

 is soon reached, while some salt still remains undissolved at the bottom of the vessel. 

 It is then continually stirred ; the temperature rises slowly while the salt gradually 

 passes into solution, until, at a certain temperature, the amount of salt remaining 

 undissolved is such that a further rise of one degree of temperature will evidently 

 cause it to disappear. The vessel is now weighed, and, as result, we have the weight 

 of salt dissolved in 25 grams of water at about the last observed temperature. AVith 

 a little care it is easy to arrange that this temperature shall be in the neighbourhood 

 of that of the air. The vessel with its contents is now heated, and salt added by 

 degrees, while the temperature rises and finally reaches the boiling point or whatever 

 other temperature may have been determined on. Salt is added until the liquid is 

 saturated at this temperature. The vessel is again weighed and the salt dissolved 

 at the higher temperature is ascertained. These simple experiments, which are com- 

 pleted in very few minutes, furnish all the information that is required for the 

 economical employment of the material. In the absence of more detailed information, 

 the following results obtained in the above way are worth quoting : — 



